In recent years there have been an increasing number of reports on the use of conjugated molecular materials as the emissive layers in electroluminescent (EL) devices; C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913; C. W. Tang, S. A. VanSlyke and C. H. Chen, J. Appl. Phys. 1989, 65, 3610; C. Adachi, S. Tokito, T. Tsutsui and S. Saito, Jpn. J. Appl. Phys. 1988, 27, 59; C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett. 1989, 55, 1489; C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lent. 1989, 56, 799; M. Nohara, M. Hasegawa , C. Hosokawa, H. Tokailit and T. Kusomoto, Chem. Lett. 1990, 189. These materials have efficient light output and have been shown to be easily amenable to excellent colour variation.
A considerable number of blue light-emitting molecular compounds for electroluminescent devices have been investigated so far. C. Adachi, T. Tsutsui and S. Salvo report a set of examples consisting of anthracene, 9,10-diphenyianthracene, 1,1,4,4-tetraphenylbutadiene, substituted distyrylbenzenes, terphenyl, 1,8-naphthalimides, naphthalene-1,4,5,8,-tetracarboximide, 2,5-bis (5-tert-butylbenzoxazol-2-yl)thiophene, tetraphenylcyclopentadiene, pentaphenyicyciopentadiene and substituted stilbenes in "Blue light-emitting organic electroluminescent devices" published in Appl. Phys. Lett., 1990, 56, 799. Furthermore Eur. Pat. Appl. EP 319 881, EP 373 582, EP 388 768 (Idemitsu Kosan Co. Appl. EP 319 881, EP 373 582, EP 388 768 (Idemitsu Kosan Co. Ltd.) neach that distyrylbenzenes are useful organic compounds for molecular electroluminescent devices. Green, blue-green and blue light emission was observed after varying the substituents on the distyrylbenzene. Other blue light-emitting compounds are quaterphenyl, as mentioned in Eur. Pat. Appl. EP 387 715 (idemitsu Kosan Co. Ltd.), and oligothiophenes, as disclosed in Eur. Pat. Appl. EP 439 627 (Ideminsu Kosan Co. Ltd.). Recently 1,3,4-oxadiazole dimers have been shown to give rise to bright blue emission in molecular electroluminescent devices: Y. Hamada, C. Adachi, T. Tsunsui and S. Saito, Optoelectronics--Devices and Technologies, 1992, 7, 83 and Y. Hamada, C. Adachi, T. Tsutsui and S. Saino, Jpn. J. Appl. Phys., 1992, 31, 1812.
However, amorphous films, as obtained by sublimation and used in all these devices, tend to undergo changes in morphology due to the heat produced under the device operating conditions. This results in the breakdown of molecular electroluminescent devices after longterm storage or even earlier during continuous driving tests.
Working with electron-transporting layers in association with electroluminescent devices which have polymeric light-emitting layers the present applicants found that a uniform dispersion of tert-butyl-PBD (1) in poly(methyl methacrylate) (PMMA) could be formed. The results are reported in "Light-emitting diodes based on conjugated polymers: Control of colour and efficiency", P. L. Burn, A. B. Holmes, A. Kraft, A. R. Brown, D. D. C. Bradley and R. H. Friend in: Electrical, optical, and magnetic properties of organic solid state materials: symposium held Dec. 2-6, 1991, Boston, Mass., USA, L. Y. Chiang, A. F. Garito, D. J. Sandman (Eds.), Pittsburgh, 1992, Mat. Res. Soc. Proc. 1992, 247, 647. Films were obtained by spin-coating a solution of (1) and PMMA in chloroform and were found to be stable under the device operating conditions. ##STR1## Dispersion-type electroluminescent devices have been reported in Jpn. Kokai Tokkyo Koho JP 03/147,290 (Hitachi Chemical Co. Led.); Jpn. Kokai Koho-yo Koho JP 03/119,090 (NEC Corp.); Jpn. Kokai Tokkyo Koho JP 03/250,582 (Hitachi Maxell Ltd.); U.S. Pat. No. 5,075,172 (Cape Cod Research Inc.). "Molecularly doped polymers as a hole transpot: layer in organic electroluminescent devices" teaches the application of a dispersion of N, N'-diphenyt-N,N'-bis(3-methylphenyl)benzidine in polycarbonate as the hole transport layer and tris(8-quinoiinato)aluminum as the emitting layer and is published by J. Kido, K. Hongawa, M. Kohda, K. Nagai, K. Okuyama, Jpn. J. Appl. Phys., 1992, 31, L960. "A light-emitting diode using as recombination layer a blend containing a poly(paraphenylene vinylene) oligomer" has been disclosed by W. Tachetet and H. J. Geise at the International Conference on Science and Technology of Synthetic Metals in Gothenburg, Sweden (Aug. 12-18, 1992). A light-emitting diode has been constructed using 1,4-bis[2-(3,4,5-trimethoxypheny)ethyl)-2,5-dimethoxybenzene blended in polystyrene as the light-emitting layer. According to the authors the advantage of such a blend over a pure oligomer was seen in a combination of mechanical and electro-optical properties which was not easily attainable with the presently known pure electro-active compounds.
Currently the patent literature contains only occasional reports on the use of potential electroluminescent dyes which are attached to a polymer in form of a side chain. Euro Pat. Appl. EP 390 551 (Kabushiki Kaishi Toshiba) discloses a way of introducing light-emitting organic dyes in the repeating unit of main and side chain oligomers with standard linking units.
Recently a number of patents appeared dealing with using polymers as luminescent substances in organic EL devices. Example of such polymers are poly(vinylanthracene), Jpn. Kokai Tokyo Koho JP 03/020 992 and 03/037 991 (Seiko Epson Corp.), and poly(vinylanthracene)/poly(styrene) copolymers, Jpn. Kokai Tokkyo Koho JP 03/002 209 (Idemitsu Kosan Co. Ltd.); it should be noted that during radical polymerisation of vinylanthracene the anthracene ring system does not stay intact and the aromatic ring is not a side chain in the final polymer but is incorporated into the main chain. Ferroelectric liquid crystalline poly(methacrylate)s have been disclosed as dispersants for light-emitting substances, such as perylene, Jpn. Kokai Tokkyo Koho JP 03/043 991 (Canon KK). Polymers of type (--Ar--O--C.sub.6 H.sub.4 --CH(Ar')--C.sub.6 H.sub.4 --O--).sub.n, Jpn. Kokai Tokkyo Koho JP 03/163 185 (Idemitsu Kosan Co. Ltd.), and (co)polymers of vinyl compound monomers having aromatic substituents, Jpn. Kokai Tokkyo Koho JP 03/137 186 (Asahi Chemical Ind. Co. Ltd.), are further examples of prior art.
Conjugated blue light-emitting polymers have also been disclosed. "Synthesis of a segmented conjugated polymer chain giving a blue-shifted electroluminescence and improved efficiency" by P. L. Burn, A. B. Holmes, A. Kraft, D. D.C. Bradley, A. R. Brown and R. H. Friend, J. Chem. Soc., Chem. Commun., 1992, 32 described the preparation of a light-emitting polymer that had conjugated and non-conjugated sequences in the main chain and exhibited blue-green electroluminescence with an emission maximum at 508 nm. Blue light-emission was observed in two other conjugated polymers. Poly(p-phenylene) sandwiched between indium-tin oxide and aluminium contacts has been published by G. Grem, G. Leditzky, B. Ullrich and G. Leising in Adv. Mater., 1992, 4, 36. Similarly, Y. Ohmori, M. Uchida, K. Muro and K. Yoshino reported on "Blue electroluminescent diodes utilizing poly(alkylfluorene)" in Jpn. J. Appl. Phys., 1991, 30, L1941.
Although there are already reports of limited and highly specialised prior art, there is still a need for electroluminescent polymers with a chemically tunable blue light emission.